It has been a common design practice to provide sliding contact arrangements (e.g., metal brushes) to pass current between the rotating members of electrical machinery and the stationary members, including stators and input or output terminals. Motors and generators are exemplary. In the case of large homopolar machines it has been necessary to provide a large number of sliding contacts in order to carry necessary levels of current into or out of the machine. In early designs of homopolar machines mercury had been used as a contact. Due to the toxic nature of mercury it has become common for all contacts to be mechanical in nature, resulting in friction and wear. Due to friction, sliding contacts also impose limits on the rotational speed of the motor/generator. It is desirable to provide an arrangement for transmitting electrical current between rotating and stationary components while reducing or eliminating friction and wear.
In operation of low temperature circuitry, e.g., including superconducting coils, it is necessary to conduct electrical current from an ambient temperature level (e.g., a typical room-temperature or the ambient temperature in an outdoor environment) into a low temperature environment present in regions that are typically cooled with cryogenic systems. The temperature difference between the room temperature and the cryogenic temperature in such applications can easily exceed 250 degree Kelvin and approaches 300 degrees Kelvin. The currents that need to be transferred from the relatively warm environment into the low temperature environment can range from milliamperes to many thousands of amperes. In the past, continuous copper rod conductors have been used to transfer the current from the warm environment into the low temperature environment. One end of such a conductor is permitted to be in the relatively warm state, typically at room temperature while a second end of the same conductor is maintained at the low temperature. The performance of such cooled current leads is governed by the relationship between electrical and thermal conductivity and their dependence on temperature. That is, electrical and thermal conductivity of metals increase with decreasing temperature. Accordingly, most heat in a current lead is generated in the high temperature region while less heat is generated in the cold temperature region. For a given current to be transferred through a conductive wire, the cross section of the wire can be optimized such that the temperature gradient along the wire matches the temperature difference between the high and low temperature regions while the temperature of the end of the wire which is at the room temperature end matches that of the room temperature environment.
A disadvantage associated with such conduction-cooled current leads stems from the high thermal conductivity of metals. The relationship between thermal conductivity, k, and electrical conductivity, σ is given by the Wiedemann-Franz law,
      k    σ    =            L      o        *    T  where the ratio of electrical conductivity and thermal conductivity is proportional to the temperature with an almost constant proportionality factor Lo. For a given temperature, the thermal conductivity is therefore proportional to the electrical conductivity. Furthermore, the Lorentz number Lo is about equal for all metals. As a result of the Wiedemann-Franz law, all current lead cooling based on the above-described concept further introduces significant amounts of heat into the low temperature reservoir into which the electrical current is being transferred. Due to the Carnot efficiency as well as the inefficiency of any refrigeration system, removal of any such heat, introduced into the low temperature reservoir requires a much larger amount of energy to remove it. When maintaining a reservoir at a cryogenic temperature the resulting energy penalty factor is often as high as several hundred. Consequently, several hundred Watts of power may be required to remove 1 Watt of heat at, for example, a reservoir operating at a few degrees Kelvin. Heat transferred through current leads constitutes a primary heat load in the reservoir. This is a major impediment to efficient cryogenic cooling of electrical systems. Prior efforts to avoid the consequences of the Wiedemann-Franz law in cryogenically cooled systems have attempted to reduce the heat load caused by electrical current leads. This can be accomplished by gas-cooling of the conductive wires or through use of hybrid conductive wiring. Hybrid conductive wiring uses high temperature superconductors in the transition region between the high and low temperature environments because superconducting wiring, having zero resistance, generates no heat when current propagates through the wiring into the low temperature environment.